This research project is designed to establish low toxicity approaches for maintaining mixed chimerism in beta-thalassemic mouse with the ultimate goal of developing myeloablative regimens to treat humans with congenital hemoglobinopathies. It will utilize 3 strategies developed in our laboratory: 1) murine model of in utero transplantation in beta-thalassemic mice, 2) prenatal tolerance induction and 3) high dose postnatal boots. The In utero transplantation provides excellent opportunity to introduce allogeneic cells into the developing fetal immune system with the goal of inducing tolerance to these cells From the experiments performed in different labs, it appears that the kind and relative number of cells injected defines the degree of tolerance induced. We are therefore proposing a research program in which 3 aims will be addressed: 1) establishment of optimal cell preparation for induction of high degree of tolerance following in utero transplantation; 2) establishment of strategies for maintenance of mixed chimerism. Our hypothesis is that lack of potent antigen presenting cells such as dendritic cells as well as specific B/T cell ratio in injected population are important factors for induction of high degree of tolerance. Specifically designed experiments aimed t induction of tolerance will be performed. Depending on the degree of tolerance induced, mini-myeloablative regimens will be designed and minimal level of microchimerism sufficient for correction of hemolytic anemia will be established. Additionally, protocols for postnatal boots to maintain the level of chimerism will be designed. This research will lead directly to the establishment of successful human protocols for early treatment of hemoglobinopathies in non-toxic and inexpensive manner.